The present invention relates to the preparation and use of modified zeolite catalyst compositions which are especially suitable for the conversion of aromatic hydrocarbons to provide product mixtures enriched in the para-(or 1,4-) dialkyl substituted benzene isomer.
Production of dialkyl substituted benzene compounds via alkylation, transalkylation or disproportionation of aromatic hydrocarbons is an important step in a number of commercial chemical manufacturing processes. Such reactions can be carried out over a variety of catalyst materials. Alkylation of aromatic hydrocarbons utilizing crystalline aluminosilicate catalysts has, for example, been described. U.S. Pat. No. 2,904,697 to Mattox refers to alkylation of aromatic hydrocarbons with an olefin in the presence of a crystalline metallic aluminosilicate having uniform openings of about 6 to 15 Angstrom units. U.S. Pat. No. 3,251,897 to Wise describes alkylation of aromatic hydrocarbons in the presence of X or Y-type crystalline aluminosilicate zeolites, specifically such type zeolites wherein the cation is rare earth and/or hydrogen. U.S. Pat. No. 3,751,504 to Keown et al. and U.S. Pat. No. 3,751,506 to Burress describe vapor phase alkylation of aromatic hydrocarbons with olefins, e.g., benzene with ethylene, in the presence of a ZSM-5 type zeolite catalyst.
The disproportionation of aromatic hydrocarbons in the presence of zeolite catalysts has been described by Grandio et al in the Oil and Gas Journal, Vol. 69, No. 48 (1971), U.S. Pat. Nos. 3,126,422; 3,413,374, 3,598,878; 3,598,879 and 3,607,961 show vapor-phase disproportionation of toluene over various catalysts.
In many of these prior art processes, the dialkylbenzene product produced frequently contains more of the 1,3 isomer than of the other two isomers. For example, xylene produced via the conventional catalytic methylation of toluene can have the equilibrium composition of approximately 24 percent of 1,4-, 54 percent of 1,3- and 22 percent of 1,2-isomer. Of the dialkylbenzene isomers, 1,3-dialkylbenzene is often the least desired product, with 1,2- and 1,4-dialkylbenzene being the more useful products. 1,4-Dimethylbenzene, for example, is of particular value, being useful in the manufacture of terephthalic acid which is an intermediate in the manufacture of synthetic fibers such as "Dacron". Furthermore, 1,4-methylethylbenzene, i.e., para-ethyltoluene (PET), is useful for subsequent conversion to para-methylstyrene, and for this purpose ethyltoluene products containing as much as 97% of the para isomer can be required.
Mixtures of dialkylbenzene isomers, either alone or in further admixture with ethylbenzene, have previously been separated by expensive superfractionation and multistage refrigeration steps. Such processes, as will be realized, involve high operation costs and have a limited yield. Alternatively, various modified zeolite catalysts have been developed to alkylate toluene with a greater or lesser degree of selectivity to 1,4-dialkylbenzene isomers. Hence, U.S. Pat. Nos. 3,972,832, 4,034,053, 4,128,592, and 4,137,195 disclose particular zeolite catalysts which have been treated with compounds of phosphorus and/or magnesium to increase para-selectivity of the catalysts. Para-selective boron-containing zeolites are shown in U.S. Pat. No. 4,067,920 and para-selective, antimony-containing zeolites in U.S. Pat. No. 3,979,472. Similarly, U.S. Pat. Nos. 3,965,208; 4,117,026; 4,259,537; 4,260,843; 4,275,256; 4,276,437; 4,276,438; 4,278,827 and 4,288,647 all disclose other zeolites modified with various oxides to improve catalyst para-selectivity.
Even though catalyst treatment procedures have been developed to render zeolite catalysts highly para-selective for aromatics conversion, aromatics conversion processes employing such catalysts, and especially such processes conducted on a commercial scale, generally tend to have a deselectivating effect on the catalyst. Contaminants such as moisture, metals and/or halogen introduced into the catalyst bed with the feed or with diluents can markedly lower catalyst para-selectivity. Water formed in the catalyst bed as a reaction product of the hydrocarbon conversion reactions which occur therein (e.g. when a methanol reactant is employed) can also adversely affect catalyst para-selectivity. There is thus a continuing need to develop not only aromatic conversion catalysts which have high initial para-selectivity, but also catalyst treatment procedures which are useful for restoring diminished catalyst para-selectivity and reducing catalyst susceptibility to subsequent deselectivation.
Accordingly, it is an object of the present invention to provide methods for treating para-selective zeolite-based aromatics conversion catalysts to enhance their initial or subsequently diminished para-selectivity characteristics or to reduce catalyst susceptibility to subsequent deselectivation.
It is a further object of the present invention to provide modified zeolite catalyst compositions which effectively promote the conversion of aromatics to produce mixtures containing an exceptionally high percentage, e.g., 80% by weight or more for alkylation of toluene, of para-dialkylbenzene isomer.
It is a further object of the present invention to provide highly para-selective aromatics conversion processes employing the modified zeolite catalysts described herein.